The present invention relates to a process for producing erythro-3-amino-2-hydroxybutyric acid derivatives and esters thereof, which are useful for producing medicines, agricultural chemicals and the like, and relates to a process for producing erythro-3-amino-2-hydroxybutyronitrile derivatives, which are synthetic intermediates thereof.
3-Amino-2-hydroxybutyronitrile derivatives and 3-amino-2-hydroxybutyric acid derivatives, which are derived from them, are used as synthetic intermediates of medicines, agricultural chemicals and the like, and many processes for producing them have been reported. Among them, as processes for producing nitrile derivatives by erythro selective cyanation of xcex1-aminoaldehyde derivatives, there have been disclosed (1) a process wherein N,N-dibenzylamino-L-phenylalaninal is reacted with trimethylsilyl cyanide in the presence of boron trifluoride etherate or zinc chloride in methylene chloride (Tetrahedron Lett., 29, 3295 (1988), WO 95/14653), (2) a process wherein aminoaldehyde derivatives are reacted with cyanohydrin derivatives in the presence of a metal compound, a base or an acid (Japanese Laid-open Patent Publication No. 231280/1998) and the like.
However, these processes for synthesis have the following problems in view of industrialization. Namely, the process (1) is unfit for large-scale synthesis since trimethylsilyl cyanide to be used as a cyanation agent is expensive and it is necessary to adjust reaction temperature to xe2x88x9210xc2x0 C. or lower in order to obtain high selectivity. The process (2) is also unfit for large-scale synthesis since expensive aluminium reagents such as dichloroethylaluminium and triisobutylaluminium are required in order to carry out the reaction in high erythro selectivity.
Doing studies precisely to solve the above-mentioned problems, the present inventors found a process wherein erythro 3-amino-2-hydroxybutyric acid derivatives having desired configuration can be obtained simply and selectively using 2-aminoaldehyde derivatives as raw materials to accomplish the present invention.
A The present invention provides a process for producing an erythro-3-amino-2-hydroxybutyric acid derivative characterized in that a 2-aminoaldehyde derivative represented by the general formula [I]
(wherein R1 is a straight-chain, branched or cyclic alkyl group having one to six carbon atoms, an alkylthio group or an arylthio group having one to eight carbon atoms, or a substituted or unsubstituted aryl group, P1 and P2 are, the same or different, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aralkyloxycarbonyl group, a substituted or unsubstituted arylcarbonyl group, or a substituted or unsubstituted arylsulfonyl group, and P1 and P2 can join each other to form a substituted or unsubstituted phthaloyl or naphthaloyl ring)
is reacted with a metal cyanide in the presence of an acid chloride and/or an acid anhydride or reacted with an organic cyanide in the presence of a Lewis acid to give stereoselectively an erythro-3-amino-2-hydroxybutyronitrile derivative represented by the general formula [II] 
(wherein R1, P1 and P2 are the same as mentioned above, and R2 is an alkylcarbonyl group, or a substituted or unsubstituted arylcarbonyl group), and then the nitrile derivative is treated with an acid in water or in a water-containing solvent to convert it into an erythro-3-amino-2-hydroxybutyric acid derivative represented by the general formula [III]
(wherein R1 is the same as mentioned above, R3 is hydrogen, Q1 and Q2 are, the same or different, hydrogen, a substituted or unsubstituted aralkyl group, or a substituted or unsubstituted arylsulfonyl group, and Q1 and Q2 can join each other to form a substituted or unsubstituted phthaloyl or naphthaloyl ring),
or the nitrile derivative is treated with an acid in an alcoholic solvent represented by the general formula of R3OH to convert it into an ester of the butyric acid represented by the above general formula [III] (in each formula, R1, Q1 and Q2 are the same as mentioned above, and R3 is a straight-chain, branched or cyclic alkyl group having one to six carbon atoms, or a substituted or unsubstituted aralkyl group).
The reaction steps of the present invention are illustrated by the following reaction formula. 
First, the first step, namely the cyanation is described.
When the 2-aminoaldehyde derivative whose amino group is protected by the bulky substituent represented by the formula [I] is cyanated, the 3-amino-2-hydroxybutyronitrile derivative [II] having erythro configuration is stereoselectively obtained. This cyanation is classified into the following two processes depending on the kind of cyanation agent to be used.
(a) When the cyanation agent is the metal cyanide, the reaction is carried out in the presence of the acid chloride and/or the acid anhydride in a two-phase solvent containing a phase-transfer catalyst.
(b) When the cyanation agent is the organic cyanide, the reaction is carried out in the presence of the Lewis acid in an aprotic solvent.
Examples of the metal cyanide to be used in the process (a) are sodium cyanide, potassium cyanide, magnesium cyanide, silver cyanide, copper cyanide and the like. Sodium cyanide and potassium cyanide are preferably used. An amount of the metal cyanide to be used is preferably one to three equivalents, more preferably one to two equivalents to a substrate (i.e. the 2-aminoaldehyde derivative [II], the same definition is applied hereinafter). Use of an excess metal cyanide does not affect a yield, but it is economically disadvantageous.
The acid chloride and/or the acid anhydride existing in the solvent in the process (a) acts as a capturing agent of alkoxide oxygen formed when the metal cyanide is added to the aldehyde as shown in the later reaction formula (A). Examples of the acid chloride are acetyl chloride, acetyl bromide, propionyl chloride, propionyl bromide, valeryl chloride, t-butylacetyl chloride, trimethylacetyl chloride, benzoyl chloride, benzoyl bromide, p-toluyl chloride, p-anisoyl chloride and the like. Acetyl chloride and benzoyl chloride are preferably used. Examples of the acid anhydride are acetic anhydride, propionic anhydride, butyric anhydride, valeric anhydride, isovaleric anhydride, trimethylacetic anhydride, benzoic anhydride, p-toluic anhydride, p-anisic anhydride and the like. Acetic anhydride and benzoic anhydride are preferably used. An amount of the acid chloride and/or the acid anhydride to be used is preferably one to three equivalents, more preferably one to two equivalents to the substrate.
Unless alkoxide oxygen of a reactive intermediate formed by stereoselective addition of the cyanide to the aldehyde, i.e. the erythro-3-amino-2-hydroxybutyronitrile derivative is successively captured immediately after generation, addition of the cyanide to the aldehyde and elimination of the cyanide from the above-mentioned reactive intermediate, which is the resulting adduct, occur reversibly and racemization proceeds remarkably as shown in the following reaction formula (B). Accordingly, it is effective to add the capturing agent in order to obtain the product in high erythro configuration selectivity. 
A preferred reaction solvent of the process (a) is the two-phase solvent consisting of water and a water-insoluble organic solvent. Examples of the water-insoluble organic solvent are hydrocarbon solvents such as n-hexane, benzene and toluene; ester solvents such as methyl acetate, ethyl acetate, propyl acetate and butyl acetate; ether solvents such as diethyl ether, methyl t-butyl ether and ethyl t-butyl ether; halogen solvents such as dichloromethane, chloroform and 1,2-dichloroethane; and mixed solvents thereof. Ethyl acetate, toluene, dichloromethane, 1,2-dichloroethane and methyl t-butyl ether are preferably used.
Reaction temperature of the process (a) is preferably 0 to 50xc2x0 C., more preferably 0xc2x0 to 25xc2x0 C. The reaction of the process (a) is usually carried out under ordinary pressure and can also be carried out under elevated pressure.
The reaction of the process (a) proceeds without a catalyst, but the reaction is promoted by adding the phase-transfer catalyst. Examples of the phase-transfer catalyst are quaternary ammonium salts such as tetrabutylammonium chloride, tetrabutylammonium bromide, benzyltrimethylammonium bromide, benzyltriethylammonium chloride, benzyltributylammonium chloride, methyltrioctylammonium chloride, tetraoctylammonium bromide and N-benzylquinium chloride; quaternary phosphonium salts such as tetrabutylphosphonium chloride, tetrabutylphosphonium bromide, tetraphenylphosphonium chloride, tetraphenylphosphonium bromide, benzyltriphenylphosphonium chloride and benzyltriphenylphosphonium bromide; and crown ethers such as 12-crown-4, 15-crown-5 and 18-crown-6. Tetrabutylammonium bromide and benzyltributylammonium chloride are preferably used. An amount of the phase-transfer catalyst to be added is preferably 0.05 to 1.1 equivalents to the substrate.
Next, the above-mentioned process (b) is described.
Examples of the organic cyanide to be used in the process (b) are acyl cyanides such as acetyl cyanide, benzoyl cyanide and 4-methyl-2-oxopentanenitrile; and cyanoformates such as methyl cyanoformate and ethyl cyanoformate. An amount of the organic cyanide to be used is preferably one to three equivalents, more preferably one to two equivalents to the substrate. Use of an excess organic cyanide does not affect the yield, but it is economically disadvantageous.
Since the organic cyanide plays also a role as a capturing agent of alkoxide oxygen formed when the organic cyanide is added to the aldehyde in the process (b), it is unnecessary to add the capturing agent such as the acid chloride or the acid anhydride.
The Lewis acid catalyzes the present reaction of the process (b). Examples of the Lewis acid are zinc chloride, zinc bromide, boron trifluoride etherate, aluminium chloride, tin tetrachloride, titanium tetrachloride and the like. Zinc chloride and zinc bromide are preferably used. An amount of the Lewis acid to be added is preferably 0.05 to 1.1 equivalents to the substrate.
The reaction solvent of the process (b) is preferably the aprotic solvent. Examples of the solvent are hydrocarbon solvents such as n-h exane, benzene and toluene; ester solvents such as methyl acetate, ethyl acetate, propyl acetate and butyl acetate; ether solvents such as diethyl ether, methyl t-butyl ether, ethyl t-butyl ether, tetrahydrofuran, 1,4-dioxane, glyme, diglyme and triglyme; halogen solvents such as dichloromethane, chloroform and 1,2-dichloroethane; aprotic polar solvents such as N,N-dimethylformamide, dimethyl sulfoxide and hexamethylphosphoric triamide; nitrile solvents such as acetonitrile; and mixed solvents thereof. Tetrahydrofuran, 1,4-dioxane and N,N-dimethylformamide are more preferably used.
Reaction temperature of the process (b) is preferably xe2x88x9220xc2x0 to 50xc2x0 C., more preferably 0xc2x0 to 25xc2x0 C. The reaction of the process (b) is usually carried out under ordinary pressure and can also be carried out under elevated pressure.
When optically active 2-aininoaldehyde derivatives are used as raw materials of the processes (a) and (b), optically active wherein the 2-aminoaldehyde derivative is an optically active substance, and the produced erythro-3-amino-2-hydroxybutyronitrile derivative is an optically active substance.
Next, the substituents P1, P2, R1 and R2 of the compound [I] and the compound [II] in the first step are described.
The groups P1 and P2, which are protective groups of the amino group, can be the same or different. Examples of the groups are the substituted or unsubstituted aralkyl groups such as benzyl, triphenylmethyl, di(4-methoxyphenyl)methyl and (4-methoxyphenyl)diphenylmethyl groups; the substituted or unsubstituted aralkyloxycarbonyl groups such as benzyloxycarbonyl, p-methoxybenzyloxycarbonyl and p-nitrobenzyloxycarbonyl groups; the substituted or unsubstituted arylcarbonyl groups such as benzoyl, p-toluyl, p-anisoyl and p-phenylbenzoyl groups; and the substituted or unsubstituted arylsulfonyl groups such as benzenesulfonyl, p-toluenesulfonyl, 4-methoxybenzenesulfonyl and m-nitrobenzenesulfonyl groups. P1 and P2 can join each other to form the substituted or unsubstituted phthaloyl or naphthaloyl rings such as phthaloyl, 4-methylphthaloyl, 4-nitrophthaloyl, 1,8-naphthaloyl and 4-nitro-1,8-naphthaloyl groups. These groups are bulky secondary amino groups.
Examples of the substituent R1 are the straight-chain, branched or cyclic alkyl groups having one to six carbon atoms such as methyl, ethyl, isopropyl, isobutyl, t-butyl and cyclohexyl groups; the alkylthio or arylthio groups such as methylthio, ethylthio and phenylthio groups; and the substituted or unsubstituted aryl groups such as phenyl, p-methoxyphenyl, p-chlorophenyl and p-nitrophenyl groups.
The substituent R2 corresponds to an alkylcarbonyl group or an arylcarbonyl group of the acid chloride or the acid anhydride to be used as the capturing agent in the process (a) or an alkylcarbonyl group or an arylcarbonyl group of the organic cyanide to be used in the process (b). Specific examples of the substituent are the alkylcarbonyl groups such as acetyl, propionyl, valeryl, t-butylacetyl and trimethylacetyl groups, and the substituted or unsubstituted arylcarbonyl groups such as benzoyl, p-toluyl and p-anisoyl groups.
The intermediate [II] obtained in the first step can be purified and then used in the second step or can be used in the second step without purification.
Next, the second step, namely the acid treatment is described.
First, the conversion of the intermediate [II] into the carboxylic acid is described.
In order to convert the intermediate en erythro-3-amino-2-hydroxybutyronitrile derivative [II] into the corresponding carboxylic acid (corresponding to a compound whose R3 is hydrogen in the formula [III]), the above-mentioned reaction is carried out in water or the water-containing solvent using 0.5 to 12 N hydrochloric acid or 0.5 to 36 N sulfuric acid as the acid. The above-mentioned reaction can also be carried out by dissolving, suspending or emulsifying the intermediate [II] in water or the water-containing solvent and then bubbling a hydrogen chloride gas through the liquid, or adding the intermediate [II] to water or the water-containing solvent saturated with hydrogen chloride by bubbling in advance. The water-containing solvent means a mixture of water and an organic solvent. Proportion of water is not limited, and it is, for example, 20% by weight or higher, preferably 30% by weight or higher. When the intermediate [II] is water-insoluble, the organic solvent dissolves it and makes the above-mentioned reaction proceed. The water-containing solvent at least has proportion of the organic solvent to enable this action.
Examples of the organic solvent constituting the water-containing solvent are hydrocarbon solvents such as n-hexane, benzene and toluene; ester solvents such as methyl acetate, ethyl acetate, propyl acetate and butyl acetate; ether solvents such as diethyl ether, methyl t-butyl ether, ethyl t-butyl ether, tetrahydrofuran, 1,4-dioxane, glyme, diglyme and triglyme; halogen solvents such as dichloromethane, chloroform and 1,2-dichloroethane; aprotic polar solvents such as N,N-dimethylformamide, dimethyl sulfoxide and hexamethylphosphoric triamide; nitrile solvents such as acetonitrile; and mixed solvents thereof.
Next, the conversion of the intermediate into the ester is described.
In order to convert the intermediate erythro-3-amino-2-hydroxybutyronitrile derivative [II] into the ester thereof, the nitrile derivative [II] is dissolved, suspended or emulsified in the alcoholic solvent and a hydrogen chloride gas is bubbled through the liquid, or the nitrile derivative [II] is added to the alcoholic solvent saturated with hydrogen chloride in advance. The alcoholic solvent means a solvent consisting of alcohol alone or mainly alcohol.
Examples of the alcohol to be used are straight-chain, branched or cyclic alkyl alcohols having one to six carbon atoms such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, t-butyl alcohol, n-pentyl alcohol, 1-methylbutyl alcohol, 3-methylbutyl alcohol, 1,2-dimethylpropyl alcohol, 2,2-dimethylpropyl alcohol and l-ethylpropyl alcohol, and substituted or unsubstituted aralkyl alcohols such as benzyl alcohol.
Reaction temperature of the second step is not particularly limited, and it is usually preferably room temperature to 100xc2x0 C. The reaction of the second step is usually carried out under ordinary pressure and can also be carried out under elevated pressure.
After the second step is finished, the desired compound [III] can be collected by a conventional method. For example, when the solvent is evaporated from the reaction mixture, the desired compound is obtained in the form of a salt such as a hydrochloride or a sulfate. Then, the desired compound [III] can be obtained in a desalted form by adding an aqueous alkali solution and the like to the compound to neutralize it, extracting the whole with an organic solvent which is immiscible with water and evaporating the solvent. Furthermore, the desired compound [III] can be purified by distillation, recrystallization, chromatography or the like, if necessary.
When the optically active erythro-3-amino-2-hydroxybutyronitrile derivatives [II] are used as raw materials of the second step, optically active erythro-3-amino-2-hydroxybutyric acid derivatives or esters thereof [III] are obtained. In this case, remarkable racemization does not occur during the reaction.
The substituents Q1, Q2 and R3 of the erythro-3-amino-2-hydroxybutyric acid derivative or the ester thereof [III] are described.
The substituent R3 is hydrogen, the straight-chain, branched or cyclic alkyl group having one to six carbon atoms, or the substituted or unsubstituted aralkyl group. The alkyl group or the aralkyl group corresponds to an alkyl site or an aralkyl site of the alcohol R3OH to be used as the reaction solvent.
The substituents Q1 and Q2 can be the same or different. When the substituents P1 and P2 of the nitrile derivative [II] remain as they are without elimination even by the above-mentioned acid treatment, Q1 and Q2 are the substituted or unsubstituted aralkyl groups such as a benzyl group; or the substituted or unsubstituted arylsulfonyl groups such as benzenesulfonyl, p-toluenesulfonyl, 4-methoxybenzenesulfonyl and m-nitrobenzenesulfonyl groups. Q1 and Q2 can join each other to form the substituted or unsubstituted phthaloyl or naphthaloyl rings such as phthaloyl, 4-methylphthaloyl, 4-nitrophthaloyl, 1,8-naphthaloyl and 4-nitro-1,8-naphthaloyl groups. When the substituents P1 and P2 are eliminated by the above-mentioned acid treatment, Q1 and Q2 are hydrogen. The aralkyloxycarbonyl group and the arylcarbonyl group of the substituents P1 and P2 are eliminated by the above-mentioned acid treatment.